Allow read-only arrays as input to remap (scikit-image#7535)

In `skimage.util.map_array`, allow read-only arrays 
(`WRITEABLE` flag set to `False`) as input to the parameters
`input_arr`, `input_vals`, and `output_vals`.

Closes scikit-image#6378.

We should probably add consts across a much larger portion of our 
Cython code, but this PR illustrates how it can be done.

---------

Co-authored-by: Juan Nunez-Iglesias <jni@fastmail.com>
Co-authored-by: Lars Grüter <lagru@mailbox.org>

Author: 3 people

environment.yml

@@ -14,7 +14,7 @@ dependencies:
   # build
   - meson-python>=0.16
   - ninja>=1.11.1.1
-  - Cython>=3.0.8,!=3.2.0b1
+  - Cython>=3.0.10,!=3.2.0b1
   - pythran>=0.16
   - numpy>=2.0
   - spin==0.13

pyproject.toml

@@ -48,7 +48,7 @@ build = [
     # Also update [build-system] -> requires
     'meson-python>=0.16',
     'ninja>=1.11.1.1',
-    'Cython>=3.0.8,!=3.2.0b1',
+    'Cython>=3.0.10,!=3.2.0b1',
     'pythran>=0.16',
     'numpy>=2.0',
     # Developer UI
@@ -124,7 +124,7 @@ tracker = 'https://github.com/scikit-image/scikit-image/issues'
 build-backend = 'mesonpy'
 requires = [
     'meson-python>=0.16',
-    'Cython>=3.0.8,!=3.2.0b1',
+    'Cython>=3.0.10,!=3.2.0b1',
     'pythran>=0.16',
     'lazy_loader>=0.4',
     'numpy>=2.0',

requirements/build.txt

@@ -2,7 +2,7 @@
 # Do not edit this file; modify pyproject.toml instead.
 meson-python>=0.16
 ninja>=1.11.1.1
-Cython>=3.0.8,!=3.2.0b1
+Cython>=3.0.10,!=3.2.0b1
 pythran>=0.16
 numpy>=2.0
 spin==0.13

src/skimage/util/_remap.pyx

@@ -7,8 +7,8 @@ from .._shared.fused_numerics cimport np_numeric, np_anyint
 
 @cython.boundscheck(False)  # Deactivate bounds checking
 @cython.wraparound(False)   # Deactivate negative indexing
-def _map_array(np_anyint[:] inarr, np_numeric[:] outarr,
-               np_anyint[:] inval, np_numeric[:] outval):
+def _map_array(const np_anyint[:] inarr, np_numeric[:] outarr,
+               const np_anyint[:] inval, const np_numeric[:] outval):
     # build the map from the input and output vectors
     cdef size_t i, n_map, n_array
     cdef unordered_map[np_anyint, np_numeric] lut

tests/skimage/util/test_map_array.py

@@ -1,3 +1,5 @@
+import itertools
+
 import numpy as np
 import pytest
 from skimage.util._map_array import map_array, ArrayMap
@@ -39,6 +41,38 @@ def test_map_array_simple(dtype_in, dtype_out, out_array):
         assert out is result
 
 
+@pytest.mark.parametrize("writeable_flags", itertools.product([True, False], repeat=3))
+@pytest.mark.parametrize("dtype", _map_array_dtypes_in)
+def test_map_array_read_only(writeable_flags, dtype):
+    """Check that input arrays can be read-only, but output_arr must not be.
+
+    See https://github.com/scikit-image/scikit-image/issues/6378.
+    """
+    input_arr = np.array([0, 2, 0, 3, 4, 5, 0], dtype=dtype)
+    input_vals = np.array([1, 2, 3, 4, 6], dtype=dtype)
+    output_vals = np.array([6, 7, 8, 9, 10], dtype=dtype)
+    out = np.full(input_arr.shape, 11, dtype=dtype)
+    desired = np.array([0, 7, 0, 8, 9, 0, 0], dtype=dtype)
+
+    for arr, writeable in zip(
+        [input_arr, input_vals, output_vals], writeable_flags, strict=True
+    ):
+        arr.flags.writeable = writeable
+
+    # this should run without error
+    result = map_array(
+        input_arr=input_arr, input_vals=input_vals, output_vals=output_vals, out=out
+    )
+    np.testing.assert_array_equal(result, desired)
+
+    # but when out is read-only, it should fail
+    out.flags.writeable = False
+    with pytest.raises(ValueError, match="read-only"):
+        _ = map_array(
+            input_arr=input_arr, input_vals=input_vals, output_vals=output_vals, out=out
+        )
+
+
 def test_map_array_incorrect_output_shape():
     labels = np.random.randint(0, 5, size=(24, 25))
     out = np.empty((24, 24))